The present invention relates to power steering devices having a pump to be driven by an electric motor and adapted to assist in steering vehicles with hydraulic pressure produced by the pump.
With conventional power steering devices of the above-mentioned type having a motor-driven pump, the voltage to be applied to a d.c. motor for driving the pump is altered to control the pump relative to the speed of the vehicle.
However, when the pressure of the pump of the conventional power steering device increases during steering, the motor current increases to entail a decreased number of revolutions since the d.c. motor is in operation at a fixed voltage, consequently resulting in a reduced flow rate. This poses a problem, for example, as to the responsiveness of the power steering device. To overcome the problem, it is required to detect the flow rate of the pump for flow control whereas no simple means has been available for detecting the flow rate of the pump. Although the flow rate of the pump can be detected by detecting the number of revolutions of the motor for driving the pump, this requires use of a revolution sensor.